Potential Problems With Tight Envelope and No AC
Joseph Crawley| Posted inGreen Building Techniqueson
I’m planning on building an off grid house. I will therefore not have air conditioning and was wondering how this affects modern building practices. I plan on a sealed crawl space with insulated walls. Six inch thick walls with foil faced foam on the exterior. A ten inch cathedral roof with foil faced foam on the exterior as well. Wall caveties will be filled with a bat type insulation. I live in zone 6b in the appalachian mountain region with hot humid summers and a heating season from September to April. The building will be heated with a masonry heater. This is something I haven’t seen addressed and was hoping you guys would have some info.
Thanks
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Replies
专门为夏天,我认为你有一个好plan if you intend to use the foil faced insulation as a radiation barrier as well as insulation. That means you would have to have an air gap between it and whatever is placed peripherally to it. That works very well to keep heat out. In your area a whole house fan would not work because of the high humidity and warmer nighttime temps (I assume) in summer. Ceiling fans in all heavily used rooms is a good idea though as many of them don't use much electricity and they really make you feel cooler.
Most of the summer night time ventilation works to bring interior temperatures down although there is at least a week or two of nights were the temp stays unbearable through the night. My current home gets into the 100% humidity zone and stays there unless we are in a drought for a large part of the summer.
I guess then it depends what your sensitivities are. There are always compromises in off-grid. A whole house fan might be a good thing if you don't mind the introduction of high humidity air for the predominant times when nighttime temps are low in summer. I have one in my low humidity region and also have radiant barriers and use no AC and am pretty happy even when daytime temps reach over 100F. Of course if that happens for a week straight all bets are off. It might be worth the compromise of introducing high humidity air in an off-grid house. Typically in a smallish to medium size house a WHF would only use 50 to 200 watts.
Martin has this article://m.etiketa4.com/article/how-to-design-an-off-grid-house
Unfortunately Martin does not address changes to the building itself based on a lack of AC in this article. The living off grid part I've gotten pretty used to over the last 15 years.
If you are OK with the humidity in the summer, I don't see a need to change anything. But if I were you, I'd think about some kind of solar A/C and/or dehumidification.
We ran all summer without AC last year in Zone 5. Summer time highs were upper 80s but never exceeding 90. Our blower door test came in just above 0 and we had no issues keeping the house in the 70s all summer and comfortable by not opening the windows and running the continuous run bath fans 24/7.
Thanks for all the replies! So over all there is nothing about the vapor barrier and insulation plan that you guys think would need to change based on the interior becoming very humid during the summer months?
My advice supporting your plan was only related to summer heat. On reflection I've realized that even the radiant barrier I have in my vented attic is perforated so it allows moisture through. I think your idea may need input from others on whether this plan is going to cause problems in winter by trapping excess humidity indoors and then having it condense on outer walls. There's others here that would know much more about that than me.
Joseph - You may want to consider that with current climate trends, your one or two weeks where the "temp stays unbearable through the night" may be two to three week within the next decade, and more in the following decades. Just something to think about in your long-term plans.
With that in mind what changes would you recommend?
Joseph,
I have a couple of non-expert thoughts. Crawl spaces really need to be conditioned (not just sealed). Since you are off-grid, I would eliminate the crawl space and install a slab or put the home on helical piers.
Since water is your home's enemy, I would extra careful when treating opens where leaks might develop. Similarly, I'd want a rain screen behind the siding to facilitate drying.
I'd also opt for air permeable insulation in the walls and ceiling and install all drywall and/or interior finishes in an airtight fashion. I would avoid recessed lighting for this reason.
I would design the electrical so there would be no issues with adding solar panels and batteries later own. These technologies seem to only get cheaper and, at some point, it might be nice to have AC for the really humid weather (or fans if nothing else).